Leaf carbon management in slow-growing plants exposed to elevated CO2

Two slow-growing plant species (Chamaerops humilis, L. and Cycas revoluta Thunb.) were exposed to elevated CO2 conditions over a 20-month period in order to study the CO2 effect on growth, photosynthetic capacity and leaf carbon (C) management. The ambient isotopic C-13/C-12 composition (delta C-13) of the greenhouse module corresponding to elevated CO2 (800 mu mol mol(-1) CO2) conditions was changed from delta C-13 ca. -12.8 +/- 0.3 parts per thousand to ca. -19.2 +/- 0.2 parts per thousand. Exposure of these plants to elevated CO2 enhanced dry mass (DM) by 82% and 152% in Chamerops and Cycas, respectively, mainly as a consequence of increases in plant level photosynthetic rates. However, analyses of A-C-i curve parameters revealed that elevated CO2 diminished leaf photosynthetic rates of Chamaerops whereas in Cycas, no photosynthetic acclimation was detected. The fact that Chamaerops plants had a lower DM increase, together with a longer leaf C residence time and a diminished capacity to respire recently fixed C, suggests that this species was unable to increase C sink strength. Furthermore, the consequent C source/sink imbalance in Chamaerops might have induced the downregulation of Rubisco. Cycas plants were capable of avoiding photosynthetic downregulation due to a greater ability to increase C sink strength, as was confirmed by DM values, and C-12-enriched CO2 labeling data. Cycas developed the ability to respire a larger proportion of recently fixed C and to reallocate the recently fixed C away from leaves to other plant tissues. These findings suggest that leaf C management is a key factor in the responsiveness of slow-growing plants to future CO2 scenarios.